1. Field of the Invention
The present invention relates to techniques for reducing power consumed by electric equipment.
2. Description of the Related Art
A power saving mode that stops a large-capacity power supply for supplying power to motors, driving circuits, and the like and electrifies only necessary control circuits in order to reduce standby power in electric equipment such as image forming apparatuses has been proposed in the past (Japanese Patent Laid-Open No. 9-200954, Japanese Patent Laid-Open No. 2002-19232). Japanese Patent Laid-Open No. 9-200954 proposes a power saving mode that, when in a standby state, operates only an efficient power supply for supplying power to a host device and an interface controller, and stops other power supplies. Meanwhile, Japanese Patent Laid-Open No. 2002-19232 attempts to save power by reducing the output voltage of a large-capacity power supply when in a power saving mode.
Incidentally, a line filter is inserted in a power supply line of a power supply apparatus in order to reduce noise (Japanese Patent Laid-Open No. 2002-19232). Meanwhile, it is necessary to communicate with a host device such as a PC even when in a power saving mode, and thus it is necessary to continually supply power to control circuits from a commercial AC power supply. In other words, with the conventional techniques, power loss occurs at the line filter as long as power is being supplied from the commercial AC power supply, even when using the power saving mode.
A typical line filter is configured of an X-capacitor that is an inter-line capacitor, a common mode choke coil, and a Y-capacitor that is a line bypass capacitor. High-capacity X-capacitors that exceed 1 [μF] are also used in switching-mode power supplies employed in power supply circuits of image forming apparatuses. With respect to X-capacitors, safety standards require that residual voltages between plugs drop below a regulated voltage within one second from the time the plug is removed from an outlet. To achieve this, it is necessary to insert discharge resistances for rapidly discharging the charge accumulated in the X-capacitor between two power supply lines. However, discharge resistances consume an amount of power determined by resistance values and input voltages when the power supply apparatus is connected to a commercial AC power supply. Power loss from discharge resistances makes up the primary portion of power loss caused by line filters.
For example, to set the discharge constant of an X-capacitor having a capacity of 1 [μF] to one second or less, it is necessary to set the resistance value of the discharge resistance to 1 [MΩ] or less. In other words, as the capacity of the X-capacitor is increased to combat noise, it is conversely necessary to reduce the discharge resistance. The power consumed by a 1 [MΩ] discharge resistance is calculated as follows when the AC input is 240 [V].Consumed power=240×240/1000000=0.0576 [W]In this manner, a greater amount of power is consumed as the resistance value of the discharge resistance drops (that is, as the capacity of the X-capacitor increases).
In a power supply apparatus whose standby power is several watts, the power consumed by the discharge resistance can be ignored. However, products whose standby power is less than 1 [W] are appearing due to market-driven demands for power savings, various countries recently strengthening their standby power regulations, and so on. It is therefore becoming more and more difficult to ignore power consumed by discharge resistances.